1. Field of the Invention
The present invention relates to raster display systems and, more particularly, and to an improved scan velocity modulation technique for use in a raster display system.
2. Related Art
Scan velocity modulation is a well-known technique used in raster display systems, such as televisions and computer displays. Scan velocity modulation is used to compensate for the suppression of high frequency components of a video signal by a video amplifier. By compensating for the suppression of high frequency components of a video signal by a video amplifier, the sharpness of an image displayed on a raster display system is improved.
Scan velocity modulation compensates for the suppression of high frequency components of the video signal by modulating the horizontal scan velocity of an electron beam (which generates the raster by scanning horizontal lines). When scanning light areas of the raster, the horizontal scan velocity of the electron beam is decreased. By contrast, when scanning dark areas of the raster, the horizontal scan velocity of the electron beam is increased. By decreasing and increasing the horizontal scan velocity of the electron beam in this manner, the sharpness (i.e., the dark-to-light and light-to-dark transitions) of the image on the raster display is improved.
FIG. 1 is a block diagram of a conventional raster display system 100 that uses the scan velocity modulation technique. Video amplifier 120 amplifies video signal VS thereby generating an amplified video signal S11. When amplifier 120 amplifies video signal VS, the high frequency components, and especially the high amplitude/high frequency components of video signal VS are suppressed. This is because video amplifier 120 has a more limited frequency bandwidth for high amplitudes at high frequencies. In other words, the high frequency components are suppressed because of slew rate limitations of video amplifier 120. As a result, amplified video signal S11 does not include all of the high frequency components of video signal VS.
This is undesirable since amplified video signal S11 is used to intensity modulate an electron beam generated by an electron gun 124. The intensity modulated electron beam strikes a phosphor coated screen 128 causing light to be emitted. The amount of light emitted depends on the intensity of the electron beam. Since amplified video signal S11 does not include all of the high frequency components of video signal VS, the electron beam is not modulated correctly and thus the image generated on screen 128 lacks sharpness.
The scan velocity modulation technique is known in the art to compensate for the suppression of the high frequency components of video signal VS and thus improve the sharpness of the image generated on screen 128. This is accomplished by providing video signal VS to a scan velocity modulation circuit, which includes high-pass filter 112, correction circuit 114, scan velocity modulation amplifier 122, and deflection coil 126.
High-pass filter 112 receives video signal VS, removes the low frequency components from video signal VS, and outputs signal S12, which includes the high frequency components of video signal VS. Correction circuit 114 receives signal S12 and generates a correction signal S13. Correction signal S13 is used to compensate for the suppression of the high frequency components of video signal VS by video amplifier 120. Scan velocity modulation amplifier 122 amplifies correction signal S13 and outputs an amplified correction signal S14. Amplified correction signal S14 is provided to deflection coil 126 and causes the horizontal scan velocity of the electron beam to increase or decrease based on the waveform of amplified correction signal S14.
A problem with the conventional scan velocity modulation technique described above is that amplified correction signal S14 provides compensation for the high amplitude/high frequency components of video signal VS (which are suppressed by video amplifier 120) as well as the low amplitude/high frequency components of video signal VS (which are not suppressed or are suppressed by a lesser amount by video amplifier 120). Consequently, amplified correction signal S14 typically overcompensates for the low amplitude/high frequency components of video signal VS and undercompensates for the high amplitude/high frequency components of video signal VS.
If an amplified correction signal S14 is generated such that it properly compensates for the low amplitude/high frequency components of video signal VS, then amplified correction signal S14 undercompensates for the high amplitude/high frequency components of video signal VS. On the other hand, if an amplified correction signal S14 is generated such that it properly compensates for the high amplitude/high frequency components of video signal VS, then low amplitude/high frequency components are distorted due to overcompensation.
Accordingly, what is needed is an improved scan velocity modulation technique.
The present invention provides an improved scan velocity modulation technique. According to the technique, a video signal is split into a first signal and a second signal. The first signal includes low amplitude/high frequency components of the video signal, which can be properly amplified by a video amplifier. The second signal includes high amplitude/high frequency components of the video signal, which cannot be properly amplified by the video amplifier. The first signal is combined with the video signal, amplified by the video amplifier, and used to modulate the intensity of an electron beam. The second signal is amplified by a scan velocity modulation amplifier and used to modulate the horizontal scan velocity of the electron beam. As a result, the low amplitude/high frequency components of the video signal are not overcompensated for and the high amplitude/high frequency components of the video signal are not undercompensated for.
In one embodiment of the present invention, a method for modulating a horizontal scan velocity of an electron beam is disclosed. The method includes generating a first signal that includes high amplitude/high frequency components of a video signal, but does not include low amplitude/high frequency components of the video signal, and modulating the horizontal scan velocity of the electron beam using the first signal.
In another embodiment of the present invention, a circuit is disclosed. The circuit includes a scan velocity modulation processor operable to generate a first signal that includes high amplitude/high frequency components of a video signal, but does not include low amplitude/high frequency components of a video signal, wherein the first signal is used to modulate a horizontal scan velocity of an electron beam.
In another embodiment of the present invention, a method is disclosed. The method includes receiving an input signal, and splitting the input signal into a first signal and a second signal, the first signal including low amplitude/high frequency components, but not high amplitude/high frequency components, and the second signal including high amplitude/high frequency components, but not low amplitude/high frequency components, wherein the second signal is used to modulate a horizontal scan velocity of the electron beam.
In another embodiment of the present invention, a circuit is disclosed. The circuit includes a scan velocity modulation processor coupled to receive an input signal, the scan velocity modulation processor operable to split the input signal into a first signal and a second signal, the first signal including low amplitude/high frequency components, but not including high amplitude/high frequency components, and the second signal including high amplitude/high frequency components, but not including low amplitude/high frequency components, wherein the second signal is used to modulate a horizontal scan velocity of the electron beam.
Other embodiments, aspects, and advantages of the present invention will become apparent from the following descriptions and the accompanying drawings.